Structure And Composition Design And Properties Of Blended Cement Paste Based On Resistance To Chloride Ingress

With rapid development of Chinese economy and marine resources utilization,more and more infrastructures are being constructed in marine and coast environment.As a result,cement-based materials with high chloride resistance are urgently needed to enhance the durability and service life of marine infrastructures.Generally,the early-age shrinkage and then cracking risk of cement paste are reduced by incorporating large amount of supplementary cementitious materials?SCMs?,such as blast furnace slag and fly ash,meanwhile pore refinement can also be achieved due to the pozzolanic reaction of SCMs,thereby delays the ingress of harmful ions.And the stability of hydration products is improved due the consumption of Ca?OH?₂.Finally,chloride resistance of concrete can be improved significantly.However,cement-based materials with high volume SCMs usually present lower mechanical properties,especially at the early age,as lower amount of hydration products are available to densify the microstructure due to low hydraulic activity of SCMs.The pervious microstructure also limits further improvement of chloride resistance.Actually,cement-based materials with high chloride resistance are characterized with homogenous and dense microstructure,low shrinkage and cracking risk,tortuous pore structure,stable hydration products with high chloride binding capacity.The shrinkage or volume stability of cement paste is mainly linked to relative humidity,pore size,porosity,etc.?factors influencing the driving force?,and chloride diffusivity is correlated to its porosity and critical pore size in numerous literatures.However,the restrict effects of unhydrated phase,which has highest modulus in cement paste,on deformation are usually neglected.Moreover,the pore tortuosity and chloride binding capacity of hydration products are not taken into account fully during microstructurally design of blended cement to improve its chloride resistance.In the present study,the content and size of unhydrated particles,characteristics of hydration products,and pores in cement paste are intentionally designed,then the influences of hydration products/unhydrated phase on the volume deformation of cement paste was investigated,the relationship between pore tortuosity and chloride diffusivity was quantified,and factors influencing the chloride binding capacity were clarified.To obtain cement paste with target composition and microstructure,the content and particle size of cement clinker,SCMs,and functional components are arranged accurately,then blended cement with homogenous microstructure,tortuous pores,and high binding capacity hydration products was prepared.Notably,the chloride diffusion coefficient of prepared blended cement was as low as 0.36×10^-12m²/s,which is 76%lower than that of commercial blended cement for marine construction.The research significances are described in detail as follows:Ground sand particles with mono-size was added into ultra-fine Portland cement paste,as ultra-fine Portland cement nearly hydrated completely,thus ground sand can be regarded as unhydrated phase.Then effects of hydration products/unhydrated phase on volume deformation of cement paste were investigated.The relationship among“shrinkage stress-bulk modulus-autogenous shrinkage”was established based on capillary stress and upscaling calculation of modulus,then the restrict mechanism of unhydrated phase on volume deformation was clarified.To maximize the restrict efficiency of unhydrated phase,the content and size of unhydrated particles were suggested to be 30%-40%and 5-10?m.The pore characteristics of cement pastes,such as porosity,pore size distribution,and pore tortuosity,were designed by changing the particle size of Portland cement and water-to-cement ratio.A conception of“tortuosity contribution factor”was proposed to quantify the contribution of each pore size fraction to the total tortuosity of cement paste,then the chloride diffusion coefficient was evaluated based on general effective media theory and the pore tortuosity of cement paste.The small capillary pore with radius of 5-20 nm had highest contribution to the pore tortuosity of cement paste.In particular,10%increase of tortuosity led to a 17.4%drop of chloride diffusion coefficient,while 33.5%reduction of chloride diffusion coefficient can be obtained by increasing 10%relative content of small capillary pores.The composition and microstructure of hydration products were adjusted by adding metakaolin and silica fume into ultrafine Portland cement,respectively.The chloride binding capacity of cement pastes was evaluated by equilibrium method,and the contribution of physical and chemical effects on chloride binding were quantified by determining the content of Friedel's salt.The amount of physically bound chloride was not only proportional to the specific surface area of hydration products,but also affected by the Al/Si ratio of C-S-H,where a high Al/Si ratio resulted in the increase of Cl^-binding site number density.The amount of chemically bound chloride increased with the AFm content as power-like function.The chloride resistance of blended cement was modified from the above viewpoints?volumetric stability,chloride diffusivity and binding capacity?.Blast furnace slag,Portland cement clinker,and fly ash were respectively arranged in the fine?<4?m?,mid-size?4-50?m?and coarse?50-90?m?fractions,based on the pore fillability of each cementitious material and close-packing theory,and the pore tortuosity of cement paste and chloride binding capacity of hydration products were also improved by incorporating metakaolin and nano-silica.Finally,a high chloride resistance blended cement with chloride diffusion coefficient of 0.36×10^-12m²/s and 28-day compressive strength of 53.2 MPa was successfully prepared,attributing to the homogenous and dense microstructure,high pore tortuosity,significant deformation restrict effect,and well-designed hydration products with high chloride binding capacity.From the viewpoints of compositions-microstructure-properties of cement paste,the type,addition,and particle size of cementitious materials were well-designed to optimize initial packing density,refine initial pore size,and improve hydration efficiency,then blended cement with excellent chloride resistance and mechanical properties were prepared.This investigation provides a deeper insight in hydraulic activity matching of cementitious materials,microstructural optimization of cement paste,reducing ingress and enhancing binding capacity of harmful ions.The results also lay a solid foundation for preparation of high-performance blended cement,efficient utilization of cementitious materials,and improvement of erosion resistance,all of which are beneficial to the improvement of durability and service life of marine infrastructures,reducing full-life environment impact of cement-based materials,and sustainable development of cement industry,presenting tremendous economic and environmental benefits.